Rollup merge of #128683 - RalfJung:miri-ui-test-deps, r=onur-ozkan

bootstrap: clear miri's ui test deps when rustc changes

`@onur-ozkan` I hope this makes sense... it's quite hard to test this, but it did at least cause one rebuild of the right files for me, and then it doesn't seem to keep rebuilding, so at first sight this look at least better than the status quo.

src/bootstrap/src/core/build_steps/test.rs

@@ -520,6 +520,14 @@ fn run(self, builder: &Builder<'_>) {
         builder.ensure(compile::Std::new(target_compiler, host));
         let host_sysroot = builder.sysroot(target_compiler);
 
+        // Miri has its own "target dir" for ui test dependencies. Make sure it gets cleared
+        // properly when rustc changes. Similar to `Builder::cargo`, we skip this in dry runs to
+        // make sure the relevant compiler has been set up properly.
+        if !builder.config.dry_run() {
+            let ui_test_dep_dir = builder.stage_out(host_compiler, Mode::ToolStd).join("miri_ui");
+            builder.clear_if_dirty(&ui_test_dep_dir, &builder.rustc(host_compiler));
+        }
+
         // Run `cargo test`.
         // This is with the Miri crate, so it uses the host compiler.
         let mut cargo = tool::prepare_tool_cargo(